Mom’s cat needs to be put to sleep. Mom has advanced dementia. Is it better to tell her or not?

No offense, but your family are idiots for wanting to have the vet come to moms house and put her cat down in front of her.
Just take her cat to the vets and have it put down there, and if mom asks about her cat, you just tell her that he/she must be hiding in the basement. And you tell her again and again that same answer anytime she asks.
And you can always buy her one of those life like battery operated cats that she can have sit on her lap and will purr when she rubs it.

Family is not thinking of what's best for mom. Quietly do what has to be done away from the house and let mom be spared the pain of losing her pet. If she asks where her pet is just be vague and redirect her,

Don’t create a problem where there isn’t one. If she doesn’t see the cat much I wouldn’t tell her. Euthanize the cat at the vet’s office.

Her family doesn't understand dementia. At all.

I definitely wouldnt tell her. Or let her be thee.

What do you think? Does it sound like a good idea to you to tell your mother with dementia that the cat is going to be put to sleep then have her watch this procedure?

No. Your mother should not be told and certainly she should not watch.

It never fails to shock me when grown adults have to ask a question like this. Really it does.

All my Cats had to be put to sleep. And each one I was with when the needle was given at the vets. Its not something that an old lady with Dementia should be forced to watch. He probably is keeping to the basement because he is dying. Animals go off to die. Just take him to the vet and if she asks about the cat, tell her he must be in the basement.

Curious, why does your Mom's cat need to be put to sleep? Of course he is hiding in the basement as the cat no longer recognizes his/her owner due to your Mom's advanced dementia. Your Mom has become a stranger to him.

Have a Vet check the cat for any illnesses, then take the cat to a local no-kill animal shelter or cat rescue group. Even senior cats find new homes. Many years ago we adopted a 12 year old cat from the shelter. Our current one was 9 when he came here to live.

I'm sorry you and your Mom had to endure this very poor decision. Who is your Mom's PoA? This is the person who really was legally responsible to make this decision. If you are the PoA, then know that this may be the first of many disagreements with your siblings over care issues. Nonetheless, the PoA does what is best for your Mom, and not what makes her children "feel good". I wish you all the best on this journey.

No! If he is at end of life and suffering, quietly have the cat euthanized without her knowledge. Leave his dish where it sits and never ever mention him. If she inquires, say something vague like “I fed him this morning.” etc. If her dementia is advanced, it is very unlikely she will process the passage of time re: his age, his health, the logic of euthanizing him, and may think the cat is being murdered. Do not put her through such an awful experience!